Malignant histiocytosis. Light micrograph of blood cells in a case of malignant histiocytosis. Hystiocytosis is an excess number of histiocytes, cells that form an important part of the immune system, specifically the cells engaged in phagocytosis (engulfing and destroying foreign particles and pathogens). Two histiocytes are at centre. The other cells are red blood cells. Malignant histiocytosis, also known as histiocytic medullary reticulosis, is a rare and hereditary form of this condition, primarily affecting the lungs and lymph nodes.